Patch organization and resilience of dryland wetlands.

Dryland wetlands are ecosystems of high ecological importance as they serve as habitat sanctuaries for aquatic and terrestrial biota in areas with very few resources; therefore, the study of such environments is of major importance for the conservation of biodiversity in arid and semi-arid areas. The vegetation organization in these ecosystems is driven by the water regime as the main driver, but local processes like seed banks and soil resources redistribution also play a crucial role in determining the spatial distribution of the vegetation. Assessment of vegetation dynamics and long-term resilience requires the use of realistic models that can integrate the water regime and that can continuously simulate vegetation extent and conditions under flood-drought cycles. Here we study the influence of the water regime as the main driver of the vegetation. We apply a vegetation-modelling framework to compare the performance of a simplified model at the cell scale and a model integrated at a patch scale. Our results show that aggregating the analysis of vegetation dynamics at the patch scale allows for the incorporation of the effects of both local drivers (acting within the patch) as well as the global drivers (acting over the patch as a whole). The water regime acts as a global driver for the vegetation and indirectly affects the local drivers. Our patch scale model successfully captures wetland vegetation dynamics using the water regime as the main driver for representing changes in the vegetation and assessment of the wetland resilience under flood-drought periods.

Assessing landscape structure and pattern fragmentation in semiarid ecosystems using patch-size distributions.

Spatial vegetation patterns are recognized as sources of valuable information that can be used to infer the state and functionality of semiarid ecosystems, particularly in the context of both climate and land use change. Recent studies have suggested that the patch-size distribution of vegetation in drylands can be described using power-law metrics, and that these scale-free distributions deviate from power-law linearity with characteristic scale lengths under the effects of increasing aridity or human disturbance, providing an early sign of desertification. These findings have been questioned by several modeling approaches, which have identified the presence of characteristic scale lengths on the patch-size distribution of semiarid periodic landscapes. We analyze the relationship between fragmentation of vegetation patterns and their patch-size distributions in semiarid landscapes showing different degree of periodicity (i.e., banding). Our assessment is based on the study of vegetation patterns derived from remote sensing in a series of semiarid Australian Mulga shrublands subjected to different disturbance levels. We use the patch-size probability density and cumulative probability distribution functions from both nondirectional and downslope analyses of the vegetation patterns. Our results indicate that the shape of the patch-size distribution of vegetation changes with the methodology of analysis applied and specific landscape traits, breaking the universal applicability of the power-law metrics. Characteristic scale lengths are detected in (quasi) periodic banded ecosystems when the methodology of analysis accounts for critical landscape anisotropies, using downslope transects in the direction of flow paths. In addition, a common signal of fragmentation is observed: the largest vegetation patches become increasingly less abundant under the effects of disturbance. This effect also explains deviations from power-law behavior in disturbed vegetation which originally showed scale-free patterns. Overall, our results emphasize the complexity of structure assessment in dryland ecosystems, while recognizing the usefulness of the patch-size distribution of vegetation for monitoring semiarid ecosystems, especially through the cumulative probability distributions, which showed high sensitivity to fragmentation of the vegetation patterns. We suggest that preserving large vegetation patches is a critical task for the maintenance of the ecosystem structure and functionality.